1. Field of the Invention
The present invention relates to an amplifier with a limiter for amplifying an electric signal directly derived from an audio source, such as a reproducing device or a receiver.
2. Description of the Related Art
In an audio device according to the related art, such as a CD (compact disc) reproducing device or an FM broadcasting receiver, it is impossible to gain an electric signal for driving a speaker directly from an audio source. To cope with this, the audio device uses an audio amplifier for amplifying an electric signal derived from the audio source. In a general audio amplifier, as the signal output to the speaker increases, a distortion contained in the output signal increases. Accordingly, there is a limit in increasing the amplitude of the output signal, which is capable of driving the speaker with less distortion. When an excessive input signal is input to the audio amplifier, or a sound volume set by a sound volume adjusting knob is excessive, the amplitude of the output signal of the amplifier exceeds a limit level within which the speaker may be driven with no distortion, the output signal to the speaker is greatly distorted, and the speaker produces loud harsh sounds. An amplifier with a limiter or an amplifier with limiter is known in which to reduce such a distortion, a limiter circuit which limits an amplitude of an input signal to a speaker-driving power amplifier is provided at the input of the power amplifier.
In FIG. 6, a pre-stage amplitude limiting amplifier 101 with an ALC (auto limit control) circuit 100 as a limiter circuit is shown for an example of the amplifier with limiter. In FIG. 6, the amplifier 101 includes a pair of power amplifiers 105 which amplifies an electrical signal selectively taken out of different kinds of audio sources 102 to 104, such as a CD reproducing device, an MD (Mini Disc) reproducing device, and an FM broadcasting receiver, and the like. Those power amplifiers 105 drive speakers 106 associated therewith, respectively. The amplifier 101, which includes the power amplifiers 105, further includes a selector switch 107, ALC circuit 100, sound-volume adjusting variable resistor 108, and a fader 109. The selector switch 107 selects one of the audio sources 102 to 104, and allows a signal derived therefrom to pass therethrough. The ALC circuit 100 limits an amplitude of the electrical signal input thereto so that the output signals of the power amplifiers 105 are not greatly distorted. The sound-volume adjusting variable resistor 108 adjusts a sound volume of sounds produced from the speakers 106. The fader 109 adjusts a sound volume balance between the paired speakers 106 by adjusting a ratio of electric signals applied to the paired power amplifiers 105.
Where the amplifier 101 is used, even if an electric signal which is derived from one of the audio sources 102 to 104 and received by it is excessively large, the ALC circuit 100 operates to limit the amplitude of the electric signal. Accordingly, the output signals to the speakers are not greatly distorted, thereby preventing loud harsh sounds from being generated from the speakers in advance. The ALC circuit 100 of the amplifier 101 is provided at the pre-stage of the sound-volume adjusting variable resistor 108. In this sense, the amplifier 101 will be referred to as an “VR pre-stage amplitude limiting amplifier”.
In FIG. 7, there is shown an amplifier 110 as another example of the amplifier with limiter. In the amplifier 110 of FIG. 7, the ALC circuit 100 of the pre-stage amplitude limiting amplifier 101 is located between the post-stage of the sound-volume adjusting variable resistor 108 and the pre-stage of the fader 109, while in the pre-stage amplitude limiting amplifier 101, ALC circuit 100 is located at the pre-stage of the sound-volume adjusting variable resistor 108. Thus, even if in the amplifier 110, a large electric signal is derived from one of the audio sources 102 to 104 in a state that a sound volume adjusting level set by the sound-volume adjusting variable resistor 108 is large, an amplitude of the electric signal to be input to the power amplifiers 105 is limited by the ALC circuit 100. Accordingly, as in the pre-stage amplitude limiting amplifier 101 already described, the output signals to the speakers are not greatly distorted, thereby preventing loud harsh sounds from being generated from the speakers in advance. The ALC circuit 100 of the amplifier 110 is provided at the post-stage of the sound-volume adjusting variable resistor 108. In this sense, the amplifier 110 will be referred to as a “VR post-stage amplitude limiting amplifier”.
In FIG. 8, there is shown an amplifier 120 as yet another example of the amplifier with limiter. In the amplifier 120 of FIG. 8, the ALC circuit 100A is provided at the post-stage of the fader 109, while the ALC circuit 100 is provided at the pre-stage of the sound-volume adjusting variable resistor 108 in the pre-stage amplitude limiting amplifier 101 already described. In the ALC circuit 100A, ALC circuits 100 are provided in association with the paired power amplifiers 105, respectively. Even if in the amplifier 120, as in the amplifier 110 already described, a large electric signal comes from one of the audio sources 102 to 104 in a state that a sound volume adjusting level set by the sound-volume adjusting variable resistor 108 is large, amplitudes of the electric signals to be input to the power amplifiers 105 are limited by the ALC circuits 100. Accordingly, the output signals to the speakers are not greatly distorted, thereby preventing loud harsh sounds from being generated from the speakers in advance. The ALC circuit 100 of the amplifier 120 is provided at the post-stage of the fader 109. In this sense, the amplifier 110 will be referred to as a “fader post-stage amplitude limiting amplifier”.
In the VR pre-stage amplitude limiting amplifier, the electric signal as an original signal derived from the audio source is directly limited in amplitude. Even in such a case where a sound volume adjusting level set by the sound-volume adjusting variable resistor is small, and there is no chance that the output signals from the power amplifiers are greatly distorted, the amplitudes of the electric signals to be amplified by the power amplifiers are all compressed to be a signal of which the amplitude is within a predetermined width. This results in suppression of the dynamic range of the original signal.
In the VR post-stage amplitude limiting amplifier, the electric signal of which the sound-volume adjusting level is adjusted by the sound-volume adjusting variable resistor is amplitude-limited. Therefore, if the sound-volume adjusting level as set is sufficiently small, the dynamic range of the original signal is not impaired. When the sound-volume adjusting level set by the sound-volume adjusting variable resistor is large to some extent, and such an input signal that the maximum amplitude is considerably larger than the minimum amplitude is input to the VR post-stage amplitude limiting amplifier, the operation of the ALC circuit cannot follow up a rapid variation of the electric signal. Accordingly, when the amplitude limiting operation starts and stops, a listener feels a sense of incongruity.
To be more specific, there is a case where an electric signal having a large dynamic range is rapidly varied from a large signal level requiring the amplitude limit to a small signal level not requiring the same, or where the user rapidly reduces a sound volume. In such a case, the amplitude limiting operation of the ALC circuit is stopped in retard of a variation of the electric signal. Further, the compression operation is completely and rapidly removed in a state that substantially the same amplitude of the electric signal continues. Accordingly, an unnatural sound is generated. Conversely, in the case where the electric signal is rapidly varied from a small signal level not requiring the amplitude limit to a large signal level requiring the same, for example, when the user rapidly increases a sound volume, the amplitude limiting function of the ALC circuit starts to be active in retard of a variation of the electric signal. Also in this case, the electric signal is rapidly and greatly compressed in a state that substantially the same amplitude of the electric signal continues. Accordingly, an unnatural sound is generated. In the case of the ALC circuit in which the amplitude limiting operation is started and stopped following up only the electric signal as input, it is difficult to start or stop the amplitude limiting operation in time for an amplitude variation of the electric signal. Accordingly, it is difficult to improve the unnatural feeling of sound generated.
In the case of the fader post-stage amplitude limiting amplifier, when it receives an electric signal of a large dynamic range, the operation of the ALC circuit cannot follow up a rapid variation of the electric signal as in the VR post-stage amplitude limiting amplifier. Accordingly, when the amplitude limiting operation is started or stopped, the listener is given the unnatural feeling. Additionally, the ALC circuits which correspond in number to the power amplifiers are required. The result is that the number of parts of the whole device is increased, and assembling work and adjusting work are complicated and cumbersome.